Les bouquinistes or used book sellers are seen all along the quais, particularly near Notre Dame. They have been an installation along the Seine since the 1500s and are now regulated by the city of Paris. It is estimated that it takes 8 years on a waiting list to be assigned one of these dark green boxes renting for about 100 Euros per year. Although there are many keychains, magnets and postcards, the city requires a balance of books and souvenirs and that the "green shop" be open at least 4 days each week.
I do love old postcards and greeting cards and the quais are great places to stroll and shop.